Magnetic dipolar ordering and relaxation in the high-spin molecular cluster compound Mn6

Abstract

Few examples of magnetic systems displaying a transition to pure dipolar magnetic order are known to date. As was recently shown, within the newly discovered class of single-molecule magnets, quite attractive examples of dipolar magnetism may be found. The molecular cluster spins and thus their dipolar interaction energy can be quite high, leading to reasonably accessible ordering temperatures even for sizable intercluster distances. In favorable cases bonding between clusters in the molecular crystal is by van der Waals forces only, and no exchange paths of importance can be distinguished. An important restriction, however, is the requirement of sufficiently low crystal field anisotropy for the cluster spin, in order to prevent the occurrence of superparamagnetic blocking at temperatures above the dipolar ordering transition. This condition can be met for molecular clusters of sufficiently high symmetry, as for the Mn6 molecular cluster compound studied here. The uniaxial anisotropy of the cluster spin S=12 is as small as D/kB=0.013 K, giving a total zero-field splitting of the S=12 multiplet of 1.9 K. As a result, the electron-spin lattice relaxation time remains fast (~10–4 s) down to Tc and no blocking occurs. Magnetic specific heat and susceptibility experiments show a transition to ferromagnetic dipolar order at Tc=0.16 K. Classical Monte Carlo calculations, performed for Ising S=12 dipoles on a lattice do predict ferromagnetic ordering and account for the value of Tc as well as the shape of the observed specific heat anomaly. By applying magnetic fields up to 6 T the hyperfine contributions Chf to the specific heat arising from the 55Mn nuclei could be detected. From the time dependence of the measured Chf the nuclear-spin lattice relaxation time T1n could be determined for the same field range in the temperature region 0.2<T<0.6 K. The nuclear magnetic relaxation was further studied by high field 55Mn pulse NMR measurements of both the nuclear T1n and T2n at T=0.9 K (up to 7 T). The data are in good mutual agreement and can be well described by the theory for magnetic relaxation in highly polarized paramagnetic crystals and for dynamic nuclear polarization, which we extensively review. The experiments provide an interesting comparison with the recently investigated nuclear spin dynamics in the anisotropic single-molecule magnet Mn12-ac